1. Field of the Invention
The present invention relates to a current perpendicular to the planes (CPP) sensor with dual self-pinned AP pinned layer structures wherein the CPP sensor is either a CPP spin valve sensor or a tunnel junction sensor.
2. Description of the Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a current perpendicular to the planes (CPP) sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive or electrically nonconductive layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The sensor is located between ferromagnetic first and second shield layers. First and second leads are connected to a bottom and a top respectively of the sensor for conducting a current perpendicular to the major thin film planes (CPP) of the sensor. This is in contrast to a CIP sensor where the current is conducted in plane parallel to the major thin film planes (CIP) of the sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the aforementioned spacer layer is nonmagnetic and electrically conductive, such as copper, the current is referred to as a sense current, but when the spacer layer is nonmagnetic and electrically nonconductive, such as aluminum oxide, the current is referred to as a tunneling current. Hereinafter, the current is referred to as a perpendicular current (IP) which can be either a sense current or a tunneling current.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the perpendicular current (IP) is at a minimum and when their magnetic moments are antiparallel the resistance of the sensor to the perpendicular current (IP) is at a maximum. Changes in resistance of the sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the perpendicular current (IP) is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the sensor at minimum resistance.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic AP layers that are separated by a coupling layer with magnetic moments of the ferromagnetic AP layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located therebetween.
The first and second shield layers may engage the bottom and the top respectively of the CPP sensor so that the first and second shield layers serve as the aforementioned leads for conducting the perpendicular current through the sensor perpendicular to the major planes of the layers of the sensor. The read gap is the length of the sensor between the first and second shield layers. It should be understood that the thinner the gap length the higher the linear read bit density of the read head. This means that more bits can be read per inch along the track of a rotating magnetic disk which enables an increase in the storage capacity of the magnetic disk drive.
Assuming that the aforementioned pinning layers are platinum manganese (PtMn) each pinning layer has a thickness of approximately 150 xc3x85 which increases the read gap in a dual CPP sensor by 300 xc3x85. This seriously impacts the linear read bit density of the read head. Further, the pinning layers significantly increase the resistance R of the sensor to the perpendicular current (IP). The result is that the magnetoresistive coefficient dr/R of the sensor is decreased. The pinning layers also require extra steps in their fabrication and a setting process. After forming the sensor, the sensor is subjected to a temperature at or near a blocking temperature of the material of the pinning layer in the presence of a field which is oriented perpendicular to the ABS for the purpose of resetting the orientation of the magnetic spins of the pinning layer. The elevated temperature frees the magnetic spins of the pinning layer so that they align perpendicular to the ABS. This also aligns the magnetic moment of the pinned layer structure perpendicular to the ABS. When the read head is cooled to ambient temperature the magnetic spins of the pinning layer are fixed in the direction perpendicular to the ABS which pins the magnetic moment of the pinned layer structure perpendicular to the ABS. After resetting the pinning layer it is important that subsequent elevated temperatures and extraneous magnetic fields not disturb the setting of the pinning layer.
An aspect of the invention is to employ a dual CCP sensor for increasing the magnetoresistive coefficient dr/R of the read head. In the dual CPP sensor first and second pinned layer structures are employed with a first spacer layer between the first pinned layer structure and the free layer and a second spacer layer between the second pinned structure and the free layer. With this arrangement the magnetoresistive (MR) effect is additive on each side of the free layer to increase the magnetoresistive coefficient dr/R of the read head. In order to reduce demagnetizing fields HD from the first and second pinned layers on the free layer, each of the pinned layers is an antiparallel (AP) pinned layer structure. The first AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers (AP1) and (AP2) and the second AP pinned layer structure has another antiparallel coupling layer which is located between another first and second AP pinned layers (AP1) and (AP2). The AP pinned layers of each AP pinned layer structure have magnetic moments which are antiparallel with respect to one another because of a strong antiferromagnetic coupling therebetween. The AP pinned layer structure is filly described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. Because of the partial flux closure between the AP pinned layers of each first and second AP pinned layer structures, each AP pinned layer structure exerts a small demagnetizing field on the free layer.
The first AP pinned layer of the first AP pinned layer structure interfaces the first spacer layer and the first AP pinned layer of the second AP pinned layer structure interfaces the second spacer layer. In order for the aforementioned MR effect to be additive on each side of the free layer it is important that the AP pinned layer structures be in-phase with respect to one another. This occurs when the magnetic moments of the first AP pinned layers of the first and second AP pinned layer structures are oriented perpendicular to the ABS and parallel with respect to one another. Accordingly, when a signal field from a rotating magnetic disk rotates the free layer structure the change in resistance of the sensor due to the magnetoresistive coefficient will be additive to increase the signal output of the read head.
Another aspect of the invention is that the AP pinned layer structures are self-pinned which eliminates the aforementioned first and second pinning layers. By eliminating the pinning layers the read gap can be reduced by at least 300 xc3x85 so that the linear read bit density of the read head is increased. Further, by eliminating the pinning layers the resistance R of the sensor is decreased so that the magnetoresistive coefficient dr/R is increased. It is preferred that the first AP pinned layer of each AP pinned layer structure next to a respective spacer layer be cobalt iron (Co90Fe10) for improving the magnetoresistive coefficient dr/R of the sensor. The second AP pinned layer of each AP pinned layer structure, however, is composed of a different material for implementing the self-pinning feature. In one aspect of the invention the material of the second AP pinned layers is magnetocrystalline MC which has a high uniaxial anistotropy HK and in another aspect of the invention the material of the second AP pinned layer is highly magnetostrictive (MS) which is face centered cube (FCC). Preferred materials for the magnetocrystalline AP pinned layers, which is hexagonal closed packed, are cobalt (Co), cobalt platinum (CO75Pt25) and cobalt samarium (CO80Sm20). Preferred materials for the highly magnetostrictive MS AP pinned layers are cobalt iron (CO50Fe50) and nickel iron (Ni45Fe55). Commonly assigned U.S. Pat. No. 6,127,053 is incorporated in its entirety regarding self-pinned AP pinned layer structures.
An object of the present invention is to provide a CPP sensor with dual self-pinned AP pinned layer structures.
Another object is to provide a CPP sensor with dual self-pinned AP pinned layer structures wherein the second AP pinned layer of each AP pinned layer structure is composed of a high magnetocrystalline material MC or a high magnetostriction material MS for implementing the self-pinning function.
Still another object is to provide a CPP sensor with a single self-pinned AP pinned layer structure wherein the second AP pinned layer of the pinned layer structure is composed of a high magnetocrystalline material MC or a high magnetostriction material MS.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.